Money and public backlash are not the only issues Heathrow expansion plans face

Rachel Reeves is supposedly someone who understands tight budgets.

While she may not have all the cash she'd like to deliver her airport expansion plans, the real constraint is carbon.

And Britain's carbon budget has, to use the fiscal parlance of the Treasury, very little headroom at all.

This isn't an academic point. Under the Climate Change Act, successive governments have a legal responsibility to meet successive five-year carbon budgets.

The watchdog that oversees those budgets, the Office for Budget Responsibility (OBR) of climate if you will, is the Committee on Climate Change. Its current advice is very clear that expansion of air travel in the UK will blow the budget unless cuts are found in other sectors.

This probably explains why the chancellor announced a new plan, coming later this year, on how the government will meet its carbon budgets.

But it's very hard to see how she might do it.

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Using figures from the Department for Transport and airport operators, the growth in flight numbers as a result of more and more frequently used runways will increase UK carbon emissions by 92 million tonnes CO2 equivalent) by 2050. (The "equivalent" bit is important as flights have an additional global warming impact due to particles and water vapour they add to the atmosphere).

There's the sustainable aviation fuel mandate that requires airlines to use an increasing mix of zero carbon fuel in the kerosene burned by jets. But the mandate is already factored into our carbon budgets and assumes no airport expansion.

This all leaves a lot of extra carbon "spending" to find cuts elsewhere in the economy to cover.

So much in fact, a recent study estimated it will cancel out all the emissions saved by the government's other key strategy of clean power by 2030. So that saving isn't going to help.

The Department for Net Zero has yet to publish plans to cut emissions from gas boilers and road vehicles, but all evidence suggests they'll struggle to meet existing carbon budgets even with the most well designed and rapidly executed policies.

So what else can Rachel Reeves do?

Planting trees is popular.

It might even help deflect some of the heat she'll be getting from environmentalists after effectively blaming bats and newts for Britain's failure at building infrastructure.

But new analysis by Carbon Brief finds that a forest twice the size of greater London would have to be in the ground and growing in short order to absorb all the CO2 those busier airports would bring.

That won't leave much space for all those homes, railways and enterprise hubs Britain also desperately needs.

(c) Sky News 2025: Money and public backlash are not the only issues Heathrow expansion plans face